Hydraulic pumps/motors have applications in many industries, including the material handling, mining and manufacturing industries.
A hydraulic motor/pump can be operated in one of two ways. In one mode of operation, the input medium is pressurised hydraulic fluid, and the output is rotational motion. The process can be reversed such that rotational motion is supplied to the hydraulic motor/pump. In this second mode of operation, the hydraulic fluid is pumped from the motor/pump.
An advantage of hydraulic motors/pumps is that they typically have an excellent overall efficiency, among many other desirable characteristics.
However, many hydraulic motors/pumps suffer from a distinct disadvantage. There exists a torque-speed trade off, such that as the motor speed increases the output torque decreases, and vice versa.
Prior art hydraulic motors/pumps typically have an eccentric disc which is connected to an output shaft. A set of hydraulic cylinder and piston assemblies are positioned in a radial (also known as a “star” or “fan”) arrangement about the axis of rotation of the output shaft. Typically there are five such hydraulic cylinder assemblies.
The pistons intermittently exert a force to the edge of the eccentric disc in a coordinated fashion such that the disc is rotated. After exerting a force, the retraction of each piston is effected by the eccentric disc.
To vary the torque of the motor (in the driving mode of operation) some such motors have been fitted with a small piston between the output shaft and the centre of the eccentric disc. The eccentricity of the disc is varied by changing the length of small piston.
Similarly in a pumping mode of operation, the fluid flow rate and/or the output fluid pressure can be altered by changing the length of the small piston.
One disadvantage of such prior art hydraulic motors/pumps is that when the output shaft speed exceeds the fluid flow capabilities of the hydraulic cylinders, the pistons can dissociate from the eccentric disc. This can result in complete failure of the hydraulic motor/pump.
A further disadvantage of the prior art devices having a variable eccentric disc is that the possible range of eccentricity is limited. Typically a zero eccentricity situation is not possible.
A still further disadvantage is that the small piston can allow small unwanted perturbations of the eccentricity. These perturbations are the result of the fluid properties and the system elasticity.
With a high overall efficiency obtainable from hydraulic motors/pumps, there is a need for such a device which can simultaneously produces high torque at high speed.